The present invention relates to an electrostatic substrate-carrier that is heated and biased at high voltage.
The field of the invention is that of treating substrates in a low pressure atmosphere when the substrates are biased at high voltage.
More particularly, the invention seeks to implant ions that enable impurities to be inserted into the substrate, a technique that is known as doping. Doping serves to modify certain properties of the substrate, which may be mechanical, thermal, electrical, hydrophobic, etc.
In order to perform such implanting, it is nowadays possible to use an ion implanter that operates in plasma immersion mode. Thus, implanting ions in a substrate consists in immersing the substrate in a plasma and in biasing it with a negative voltage of several tens of volts to several tens of kilovolts (generally less than 100 kV), in order to create an electric field capable of accelerating the ions of the plasma towards the substrate so that they become implanted therein. The atoms as implanted in this way are called dopants. The bias is generally pulsed.
In certain applications, it is appropriate to heat the substrate.
Implantation machines are thus known that use infrared lamps for heating the substrate. That situation is appropriate for implanting mechanical parts by means of a plasma that can be said to be “clean” (nitrogen, oxygen, . . . ). It is not appropriate when doping silicon: the plasma used is likely to create interfering deposits on the lamps, thereby masking the radiation and disturbing heating.
Thus, implantation machines are known that make use of enclosures having hot walls. On this topic, reference may be made to the work “Handbook of plasma immersion on implantation and deposition”, year 2000, edited by André Anders, published by John Wiley & Sons, ISBN 0-471-24698-0. The exchange of heat between the hot walls and the substrate is poor when the working pressure is very low because there is practically no convection. The exchange of heat by radiation is ineffective if it is desired to raise the substrate to high temperature (higher than 300° C.) since losses are considerable. It is necessary to overdimension the heating power which leads to problems of sealing: behavior of gaskets, deformation of the mechanical components, . . . .
Implantation machines are also known that bias the substrate in alternation with a negative voltage and with a positive voltage. The negative alternation attracts ions and it is the alternation used for implanting. The positive alternation attracts electrons that transfer their energy to the substrate, thereby heating it. Temperature is controlled by acting on the duty ratio and/or by acting on the voltage used for accelerating the electrons. That solution is appropriate for treating surfaces of parts that are not sensitive. In microelectronics, and more particularly microelectronics on silicon, high-energy electrons are likely to create defects leading to a reduction in the lifetime of carriers.
Document U.S. Pat. No. 7,094,670 shows the advantage of controlling the temperature of the substrate in order to control etching or parasitic deposition when using a reactive plasma. It shows that the temperature needs to be greater than the threshold needed for depositing materials formed from the gas being used (e.g. polymers of the boron hydride type). Furthermore, the temperature must be below the threshold needed for etching the silicon substrate, regardless of whether it is polycrystalline, monocrystalline, or amorphous. In that document, the temperature of the substrate carrier is controlled by causing a heat transfer fluid to circulate at a stable temperature. The temperature of the substrate depends on the energy flux coming from the plasma and on the thermal resistance between the substrate and the substrate carrier. It follows that the temperature reached depends strongly on the method used. If the heat transfer fluid is heated, its temperature does not exceed 200° C. in practice. Thus, it is possible to reach a high temperature only if use is made of a low density plasma, and/or of a low acceleration voltage, and/or of a low implanting current.
It follows from the above that heating the substrate in the context of implanting ions by plasma immersion is a real problem. The problem is made that much more difficult if use is made of an electrostatic substrate carrier.
The advantages of such a substrate carrier are well known to the person skilled in the art and there is no need to repeat them herein.
On this topic, Document U.S. Pat. No. 6,538,872 teaches an electrostatic substrate carrier having resistive heater means. That system enables the substrate to be heated independently of the parameters of the plasma.
Nevertheless, that substrate carrier cannot be biased at high voltage given its constitution. No provision is made for isolating the various modules constituting the substrate carrier. Furthermore, the module that includes the heater resistance is inserted in a part that is in contact with the substrate carrier proper. The connection between the heater module and the substrate carrier is made by brazed bond layer in order to optimize thermal transfer. The bond layer contains metals such as tin, indium, or copper, which metals are major contaminants for ion implantation.